A polarizing plate is an apparatus configured to convert natural light into polarized light having a certain vibration direction. In recent years, polarizing plates have been applied to various display devices such as Liquid Crystal Display (LCD) devices, Organic Light Emitting Diode (OLED) devices and the like.
Such a polarizing plate is generally used in a structure in which a protective film is attached to one or both surfaces of a polarizer by means of an adhesive, wherein the polarizer is formed of a polyvinyl alcohol (hereinafter referred to as ‘PVA’)-based resin stained with a dichroic dye or iodine. In the prior art, a triacetyl cellulose (TAC)-based film has been widely used as a protective film configured to protect such a polarizing plate. However, such TAC-based films have problems in that these films may be easily deformed under conditions of high temperature and high humidity. Therefore, protective films formed of various materials, able to be substituted for such TAC films, are currently in development. For example, alternative methods of forming such protective films using polyethylene terephthalate (PET), cycloolefin polymer (COP), acrylic film and the like, alone or in combination, have been proposed.
Meanwhile, an aqueous adhesive composed of an aqueous solution of a polyvinyl alcohol-based resin has been generally used as an adhesive with which to attach such a protective film to a polarizer plate.
However, such an aqueous adhesive may have a problem in that uses thereof are limited according to materials used in film formation, since this aqueous adhesive has weak adhesive strength when an acrylic film or a COP film is used as the protective film instead of the TAC film. Also, this aqueous adhesive has problems in that when the protective films applied to both surfaces of a PVA element are formed of different materials, curling may occur in edge portions of the polarizing plate in a process of drying the aqueous adhesive, and initial optical properties may be deteriorated in addition to poor adhesive strength caused according to different types of materials used in films. Accordingly, a non-aqueous photocurable adhesive was proposed as an alternative to solve the above-listed problems.
However, in the prior art, when such a non-aqueous photocurable adhesive is applied to a double-sided polarizing plate in which protective films are attached to both surfaces of the polarizer, adhesive layers formed on both surfaces of the polarizer should be cured, leading to difficulties in processes in which both adhesive layers must be irradiated with light.
Meanwhile, to simplify a manufacturing process, when the adhesive layers formed on both surfaces of the polarizer are cured through a single irradiation process, degrees of curing of the adhesive layers may be different, due to differences in quantities of light reaching a light-irradiated surface and a non-light-irradiated surface, resulting in deteriorated adhesive strength in the non-light-irradiated surface. When the adhesive strength of the adhesive layers is deteriorated as described above, durability of the polarizing plate may be degraded, leading to deteriorated optical properties.